1. Field of the Invention
The present invention relates to a method and apparatus for implementing and managing a limited receiving memory of a hybrid automatic repeat request (HARQ) in a communication system to which a multi channel HARQ is applied.
2. Description of the Related Art
A hybrid automatic repeat request (HARQ) is a scheme in which a forward error correction (FEC) scheme and an automatic repeat request are combined. The FEC scheme uses a technique for receiving precise information by correcting an error occurring in a wireless channel using an error correcting symbol. The automatic repeat request is a scheme for requesting a transmitter from a receiver to retransmit a packet and for receiving a retransmitted packet when an error occurs in a wireless channel. In general, the HARQ scheme may attempt to correct an error with respect to a received symbol, and determine whether retransmission is to be performed using a simple error detection symbol, for example, a cyclic redundancy check (CRC). As it pertains to the retransmission, the HARQ scheme may include the following four types.
1) Type-I HARQ scheme: A receiver may discard a packet having an error, request retransmission, and a transmitter may transmit a packet identical to a packet transmitted initially.
2) Type-I HARQ scheme with Chase Combining: A method for decoding a packet having an error by combining the packet in which the error is present with a retransmitted packet in lieu of discarding the packet in which the error is present.
3) Type-II HARQ scheme (Incremental redundancy scheme): In a case of Type-I, a method for transmitting by channel coding at a high coding rate in an initial transmission to prevent an unnecessary redundancy from being transmitted during initial transmission of a packet, and gradually transmitting an additional redundancy when retransmission is requested.
4) Type-III HARQ scheme (Partial incremental redundancy scheme): A method for enhancing error correction efficiency of a receiver in which a transmitter transmits a packet by channel coding at a high coding rate during initial transmission of the packet, and transmits systematic bits and parity bits in a pattern differing from the initial transmission when retransmission is requested.
The HARQ scheme may be adopted in most mobile communication systems subsequent to a third generation (3G) system. The Type-II and type-III HARQ schemes may require a receiving buffer, or a memory, of a receiver unlike an existing automatic repeat request or the type-I HARQ scheme. In particular, an existence of a sufficient receiving buffer is required since decoding may be attempted by combining a packet being retransmitted while storing a packet initially received.
In a case of an N-Channel stop-and-wait (SAW) HARQ being used in the mobile communication system subsequent to the 3G system, since an N number of HARQ processes are transmitted simultaneously by including an initial transmission packet or a retransmission packet, a receiver may be required to have a sufficient memory for simultaneous processing of a plurality of packets transmitted through the N number of HARQ processes. In a case of a terrestrial mobile communication, an operation may be performed by eight HARQ processes due to a relatively short propagation delay time. However, in a case of a satellite communication that has a relatively long round trip delay time, approximately 500 HARQ processes may be required for geosynchronous (GEO) communication in particular, because a round trip delay time is approximately 500 milliseconds. In this instance, a relatively great receiving memory may be required in comparison to a terrestrial mobile communication system.
FIG. 1 is a diagram illustrating a size of a receiving memory required for an N number of HARQ processes according to a related art. Referring to FIG. 1, space may be required for storing a k number of packets transmitted individually for a plurality of processes when a maximum number of HARQ retransmissions is assumed to be k−1. In a case of a GEO-based satellite communication system requiring approximately 500 processes for seamless transmission, an issue in which a memory size of the receiver is required to be increased may arise, for example, to 62 times greater when compared to third generation partnership project (3GPP) long-term evolution (LTE) mobile communication system, because a memory size required for a receiver increases in proportion to a number of HARQ processes.